1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
An electrophotographic apparatus or an electrostatic recording apparatus (hereinafter, an image forming apparatus) such as a copying machine, a printer, or a facsimile includes a developing assembly for visualizing an electrostatic latent image using a non-magnetic single-component toner. Conventionally, a developing assembly which includes a developing roller as a developer bearing member for bearing and conveying toner and a supply roller disposed around the developing roller and serving as a developer supply member for supplying toner to the developing roller is known. In this developing assembly, toner is supplied to the developing roller while being triboelectrically charged by mechanical rubbing between the supply roller and the developing roller. The supplied toner, of which the thickness of a toner layer on the developing roller is regulated to a predetermined amount by a developer regulating member, is conveyed to a developing zone near a photosensitive drum, which is an electrostatic latent image bearing member, and the electrostatic latent image is visualized as a toner image.
Toner which remains on the developing roller without being used for development in the developing zone (hereinafter referred to as a “development residue toner”) is scraped off the developing roller by mechanical rubbing between the supply roller and the developing roller in a contact region contacting the supply roller. Simultaneously with this, toner is supplied from the supply roller to the developing roller. On the other hand, the scraped toner is mixed with toner present inside and near the supply roller.
Conventionally, depending on a printing pattern during an image formation period in such a developing assembly, a phenomenon in which a halftone density immediately after a background portion is different from a halftone density (hereinafter referred to as a “development ghost”) immediately after solid print may occur. The development ghost occurs due to a difference in toner charge amount which results from a difference in printing pattern and is likely to occur when the supply roller has low scraping performance.
The development ghost can be reduced when the mechanical scraping performance of the supply roller is enhanced in order to solve this problem. In this case, however, since the mechanical rubbing between the developing roller and the supply roller increases, deterioration of toner may be accelerated. If toner deterioration (that is, separation and embedding of external additives on the surface of toner) is accelerated, the degree of agglomeration may increase and charging performance may decrease. As a result, a problem such as toner filming which is melt adhesion of toner on the surface of the developing roller may occur, which may become a hindrance to extending the service life of the developing assembly. Due to this, it is desired to suppress the occurrence of development ghost using methods other than the method of enhancing mechanical rubbing.